What are the duties (functions) of Prophets (peace be upon them all)?

The author of Risale-i Nur collection Bediuzzaman with the expression, For the Qurans basic duty is to teach about the acts in the Sphere of Dominicality and the duties and situations in the Sphere of Worship points out the duties of the prophets in concise.

Allahs (swt) person, attributes, actions, and names are in the Sphere of Dominicality. And the duties such as fulfilling Allahs commands, avoiding the forbidden, and contemplating the manifestations of the Divine names and the attributes of Him are in the Sphere of Worship.

Man without adhering himself to the prophet cannot pass a decision in this field. If he does, the decision would be personal and would only satisfy his temporary cravings.

As you see, Prophets are the divine messengers who instruct the first sphere to humans and teach the duties to them which belong to the second sphere.